Enhanced interface thermoelectric coolers with all-metal tips

ABSTRACT

A thermoelectric device with improved efficiency is provided. In one embodiment, the thermoelectric device includes a first thermoelement and a second thermoelement electrically coupled to the first thermoelement. An array of first tips are in close physical proximity to, but not necessarily in physical contact with, the first thermoelement at a first set of discrete points. An array of second tips are in close physical proximity to, but not necessarily in physical contact with, the second thermoelement at a second set of discrete points. The first and second conical are constructed entirely from metal, thus reducing parasitic resistances.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is related to co-pending U.S. patentapplication Ser. No. ______ (IBM Docket No. AUS9-2000-0415-US1) entitled“THERMOELECTRIC COOLERS WITH ENHANCED STRUCTURED INTERFACES” filed______, to co-pending U.S. patent application Ser, No. ______ (IBMDocket No. AUS9-2000-0564-US1) entitled “COLD POINT DESIGN FOR EFFICIENTTHERMOELECTRIC COOLERS” filed on ______, and to co-pending U.S. patentapplication Ser. No. (IBM Docket No. AUS9-2000-0556-US1) entitled“ENHANCED INTERFACE THERMOELECTRIC COOLERS USING ETCHED THERMOELECTRICMATERIAL TIPS” filed on ______. The content of the above mentionedcommonly assigned, co-pending U. S. Patent applications are herebyincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] The present invention relates to devices for cooling substancessuch as, for example, integrated circuit chips, and more particularly,the present invention relates to thermoelectric coolers.

[0004] 2. Description of Related Art

[0005] As the speed of computers continues to increase, the amount ofheat generated by the circuits within the computers continues toincrease. For many circuits and applications, increased heat degradesthe performance of the computer. These circuits need to be cooled inorder to perform most efficiently. In many low end computers, such aspersonal computers, the computer may be cooled merely by using a fan andfins for convective cooling. However, for larger computers, such as mainframes, that perform at faster speeds and generate-much more heat, thesesolutions are not viable.

[0006] Currently, many main frames utilize vapor compression coolers tocool the computer. These vapor compression coolers perform essentiallythe same as the central air conditioning units used in many homes.However, vapor compression coolers are quite mechanically complicatedrequiring insulation and hoses that must run to various parts of themain frame in order to cool the particular areas that are mostsusceptible to decreased performance due to overheating.

[0007] A much simpler and cheaper type of cooler are thermoelectriccoolers. Thermoelectric coolers utilize a physical principle known asthe Peltier Effect, by which DC current from a power source is appliedacross two dissimilar materials causing heat to be absorbed at thejunction of the two dissimilar materials. Thus, the heat is removed froma hot substance and may be transported to a heat sink to be dissipated,thereby cooling the hot substance. Thermoelectric coolers may befabricated within an integrated circuit chip and may cool specific hotspots directly without the need for complicated mechanical systems as isrequired by vapor compression coolers.

[0008] However, current thermoelectric coolers are not as efficient asvapor compression coolers requiring more power to be expended to achievethe same amount of cooling. Furthermore, current thermoelectric coolersare not capable of cooling substances as greatly as vapor compressioncoolers. Therefore, a thermoelectric cooler with improved efficiency andcooling capacity would be desirable so that complicated vaporcompression coolers could be eliminated from small refrigerationapplications, such as, for example, main frame computers, thermalmanagement of hot chips, RF communication circuits, magnetic read/writeheads, optical and laser devices, and automobile refrigeration systems.

SUMMARY OF THE INVENTION

[0009] The present invention provides a thermoelectric device withimproved efficiency. In one embodiment, the thermoelectric deviceincludes a first thermoelement and a second thermoelement electricallycoupled to the first thermoelement. An array of first tips are in closephysical proximity to, but not necessarily in physical contact with, thefirst thermoelement at a first set of discrete points. An array ofsecond tips are in close physical proximity to, but not necessarily inphysical contact with, the second thermoelement at a second set ofdiscrete points. The first and second conical are constructed entirelyfrom metal, thus reducing parasitic resistances.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself, however, as wellas a preferred mode of use, further objectives and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

[0011]FIG. 1 depicts a high-level block diagram of a ThermoelectricCooling (TEC) device in accordance with the prior art;

[0012]FIG. 2 depicts a cross sectional view of a thermoelectric coolerwith enhanced structured interfaces in accordance with the presentinvention;

[0013]FIG. 3 depicts a planer view of thermoelectric cooler 200 in FIG.2 in accordance with the present invention;

[0014]FIGS. 4A and 4B depicts cross sectional views of tips that may beimplemented as one of tips 250 in FIG. 2 in accordance with the presentinvention;

[0015]FIG. 5 depicts a cross sectional view illustrating the temperaturefield of a tip near to a superlattice in accordance with the presentinvention;

[0016]FIG. 6 depicts a cross sectional view of a thermoelectric coolerwith enhanced structured interfaces with all metal tips in accordancewith the present invention;

[0017]FIG. 7 depicts a cross-sectional view of a sacrificial silicontemplate for forming all metal tips in accordance with the presentinvention;

[0018]FIG. 8 depicts a flowchart illustrating an exemplary method ofproducing all metal cones using a silicon sacrificial template inaccordance with the present invention;

[0019]FIG. 9 depicts a cross sectional view of all metal cones formedusing patterned photoresist in accordance with the present invention;

[0020]FIG. 10 depicts a flowchart illustrating an exemplary method offorming all metal cones using photoresist in accordance with the presentinvention;

[0021]FIG. 11 depicts a cross-sectional view of a thermoelectric coolerwith enhanced structural interfaces in which the thermoelectric materialrather than the metal conducting layer is formed into tips at theinterface in accordance with the present invention;

[0022]FIG. 12 depicts a flowchart illustrating an exemplary method offabricating a thermoelectric cooler in accordance with the presentinvention;

[0023]FIG. 13 depicts a cross-sectional diagram illustrating thepositioning of photoresist necessary to produce tips in a thermoelectricmaterial;

[0024]FIG. 14 depicts a diagram showing a cold point tip above a surfacefor use in a thermoelectric cooler illustrating the positioning of thetip relative to the surface in accordance with the present invention;and

[0025]FIG. 15 depicts a schematic diagram of a thermoelectric powergenerator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] With reference now to the figures and, in particular, withreference to FIG. 1, a high-level block diagram of a ThermoelectricCooling (TEC) device is depicted in accordance with the prior art.Thermoelectric cooling, a well known principle, is based on the PeltierEffect, by which DC current from power source 102 is applied across twodissimilar materials causing heat to be absorbed at the junction of thetwo dissimilar materials. A typical thermoelectric cooling deviceutilizes p-type semiconductor 104 and n-type semiconductor 106sandwiched between poor electrical conductors 108 that have good heatconducting properties. N-type semiconductor 106 has an excess ofelectrons, while p-type semiconductor 104 has a deficit of electrons.

[0027] As electrons move from electrical conductor 110 to n-typesemiconductor 106, the energy state of the electrons is raised due toheat energy absorbed from heat source 112. This process has the effectof transferring heat energy from heat source 112 via electron flowthrough n-type semiconductor 106 and electrical conductor 114 to heatsink 116. The electrons drop to a lower energy state and release theheat energy in electrical conductor 114.

[0028] The coefficient of performance, η, of a cooling refrigerator,such as thermoelectric cooler 100, is the ratio of the cooling capacityof the refrigerator divided by the total power consumption of therefrigerator. Thus the coefficient of performance is given by theequation:$\eta = \frac{{\alpha \quad {IT}_{c}} - {\frac{1}{2}I^{2}R} - {K\quad \Delta \quad T}}{{I^{2}R} + {\alpha \quad I\quad \Delta \quad T}}$

[0029] where the term αIT_(c) is due to the thermoelectric cooling, theterm ½I²R is due to Joule heating backflow, the term KΔT is due tothermal conduction, the term I²R is due to Joule loss, the term αIΔT isdue to work done against the Peltier voltage, α is the Seebeckcoefficient for the material, T_(c) is the temperature of the heatsource, and ΔT is the difference in the temperature of the heat sourceform the temperature of the heat sink.

[0030] The maximum coefficient of performance is derived by optimizingthe current, I, and is given by the following relation:$\eta_{\max} = {{{\left( \frac{T_{c}}{\Delta \quad T} \right)\left\lbrack \frac{\gamma - \frac{T_{h}}{T_{c}}}{\gamma + 1} \right\rbrack}\quad {where}\quad \gamma \sqrt{1 + {\frac{\alpha^{2}\sigma}{\lambda}\left( \frac{T_{h} + T_{c}}{2} \right)}}\quad {and}\quad ɛ} = \frac{\gamma - \frac{T_{h}}{T_{c}}}{\gamma + 1}}$

[0031] where ε is the efficiency factor of the refrigerator. The figureof merit, ZT, is given by the equation:${ZT} = \frac{\alpha^{2}\sigma \quad T}{\lambda}$

[0032] where λ is composed of two components: λ_(e), the component dueto electrons, and λ_(L), the component due to the lattice. Therefore,the maximum efficiency, ε, is achieved as the figure of merit, ZT,approaches infinity. The efficiency of vapor compressor refrigerators isapproximately 0.3. The efficiency of conventional thermoelectriccoolers, such as thermoelectric cooler 100 in FIG. 1, is typically lessthan 0.1. Therefore, to increase the efficiency of thermoelectriccoolers to such a range as to compete with vapor compressionrefrigerators, the figure of merit, ZT, must be increased to greaterthan 2. If a value for the figure of merit, ZT, of greater than 2 can beachieved, then the thermoelectric coolers may be staged to achieve thesame efficiency and cooling capacity as vapor compression refrigerators.

[0033] With reference to FIG. 2, a cross sectional view of athermoelectric cooler with enhanced structured interfaces is depicted inaccordance with the present invention. Thermoelectric cooler 200includes a heat source 226 from which, with current I flowing asindicated, heat is extracted and delivered to heat sink 202. Heat source226 may be thermally coupled to a substance that is desired to becooled. Heat sink 202 may be thermally coupled to devices such as, forexample, a heat pipe, fins, and/or a condensation unit to dissipate theheat removed from heat source 226 and/or further cool heat source 226.

[0034] Heat source 226 is comprised of p− type doped silicon. Heatsource 226 is thermally coupled to n+ type doped silicon regions 224 and222 of tips 250. N+ type regions 224 and 222 are electrical conductingas well as being good thermal conductors. Each of N+ type regions 224and 222 forms a reverse diode with heat source 226 such that no currentflows between heat source 226 and n+regions 224 and 222, thus providingthe electrical isolation of heat source 226 from electrical conductors218 and 220.

[0035] Heat sink 202 is comprised of p− type doped silicon. Heat sink202 is thermally coupled to n+ type doped silicon regions 204 and 206.N+ type regions 204 and 206 are electrically conducting and good thermalconductors. Each of N+ type regions 204 and 206 and heat sink 202 formsa reverse diode so that no current flows between the N+ type regions 204and 206 and heat sink 202, thus providing the electrical isolation ofheat sink 202 from electrical conductor 208. More information aboutelectrical isolation of thermoelectric coolers may be found in commonlyU.S. patent application Ser. No. 09/458,270 entitled {ElectricallyIsolated Ultra-Thin Substrates for Thermoelectric Coolers” (IBM DocketNo. AUS9-99-0413-US1) assigned to the International Business MachinesCorporation of Armonk, New York and filed on Dec. 9, 1999, the contentsof which are hereby incorporated herein for all purposes.

[0036] The need for forming reverse diodes with n+ and p− regions toelectrically isolate conductor 208 from heat sink 202 and conductors 218and 220 from heat source 226 is not needed if the heat sink 202 and heatsource 226 are constructed entirely from undoped non-electricallyconducting silicon. However, it is very difficult to ensure that thesilicon is entirely undoped. Therefore, the presence of the reversediodes provided by the n+ and p− regions ensures that heat sink 202 andheat source 226 are electrically isolated from conductors 208, 218, and220. Also, it should be noted that the same electrical isolation usingreverse diodes may be created other ways, for example, by using p+typedoped silicon and n− type doped silicon rather than the p− and n+ typesdepicted. The terms n+ and p+, as used herein, refer to highly n dopedand highly p doped semiconducting material respectively. The terms n−and p−, as used herein, mean lightly n doped and lightly p dopedsemiconducting material respectively.

[0037] Thermoelectric cooler 200 is similar in construction tothermoelectric cooler 100 in FIG. 1. However, N-type 106 and P-type 104semiconductor structural interfaces have been replaced with superlatticethermoelement structures 210 and 212 that are electrically coupled byelectrical conductor 208. Electrical conductor 208 may be formed fromplatinum (Pt) or, alternatively, from other conducting materials, suchas, for example, tungsten (W), nickel (Ni), or titanium copper nickel(Ti/Cu/Ni) metal films.

[0038] A superlattice is a structure consisting of alternating layers oftwo different semiconductor materials, each several nanometers thick.Thermoelement 210 is constructed from alternating layers of N-typesemiconducting materials and the superlattice of thermoelement 212 isconstructed from alternating layers of P-type semiconducting materials.Each of the layers of alternating materials in each of thermoelements210 and 212 is 10 nanometers (nm) thick. A superlattice of twosemiconducting materials has lower thermal conductivity, λ, and the sameelectrical conductivity, σ, as an alloy comprising the same twosemiconducting materials.

[0039] In one embodiment, superlattice thermoelement 212 comprisesalternating layers of p-type bismuth chalcogenide materials such as, forexample, alternating layers of Bi₂Te₃/Sb₂Te₃ with layers ofBi_(0.5)Sb_(1.5)Te₃, and the superlattice of thermoelement 210 comprisesalternating layers of n-type bismuth chalcogenide materials, such as,for example, alternating layers of Bi₂Te₃ with layers of Bi₂Se₃. Othertypes of semiconducting materials may be used for superlattices forthermoelements 210 and 212 as well. For example, rather than bismuthchalcogenide materials, the superlattices of thermoelements 210 and 212may be constructed from cobalt antimony skutteridite materials.

[0040] Thermoelectric cooler 200 also includes tips 250 through whichelectrical current I passes into thermoelement 212 and then fromthermoelement 210 into conductor 218. Tips 250 includes n+ typesemiconductor 222 and 224 formed into pointed conical structures with athin overcoat layer 218 and 220 of conducting material, such as, forexample, platinum (Pt). Other conducting materials that may be used inplace of platinum include, for example, tungsten (W), nickel (Ni), andtitanium copper nickel (Ti/Cu/Ni) metal films. The areas between andaround the tips 250 and thermoelectric materials 210 and 212 should beevacuated or hermetically sealed with a gas such as, for example, drynitrogen.

[0041] On the ends of tips 250 covering the conducting layers 218 and220 is a thin layer of semiconducting material 214 and 216. Layer 214 isformed from a P-type material having the same Seebeck coefficient, α, asthe nearest layer of the superlattice of thermoelement 212 to tips 250.Layer 216 is formed from an N-type material having the same Seebeckcoefficient, α, as the nearest layer of thermoelement 210 to tips 250.The P-type thermoelectric overcoat layer 214 is necessary forthermoelectric cooler 200 to function since cooling occurs in the regionnear the metal where the electrons and holes are generated. The n-typethermoelectric overcoat layer 216 is beneficial, because maximum coolingoccurs where the gradient (change) of the Seebeck coefficient ismaximum. The thermoelectric overcoat 214 for the P-type region isapproximately 60 nm thick. A specific thickness of the n− typethermoelectric overcoat 216 has yet to be fully refined, but it isanticipated that it should be in a similar thickness range to thethickness of the thermoelectric overcoat 214.

[0042] By making the electrical conductors, such as, conductors 110 inFIG. 1, into pointed tips 250 rather than a planer interface, anincrease in cooling efficiency is achieved. Lattice thermalconductivity, λ, at the point of tips 250 is very small because oflattice mismatch. For example, the thermal conductivity, λ, of bismuthchalcogenides is normally approximately 1 Watt/meter*Kelvin. However, inpointed tip structures, such as tips 250, the thermal conductivity isreduced, due to lattice mismatch at the point, to approximately 0.2Watts/meter*Kelvin. However, the electrical conductivity of thethermoelectric materials remains relatively unchanged. Therefore, thefigure of merit, ZT, may increased to greater than 2.5 for this kind ofmaterial. Another type of material that is possible for thesuperlattices of thermoelements 210 and 212 is cobalt antimonyskutteridites. These type of materials typically have a very highthermal conductivity, λ, making them normally undesirable. However, byusing the pointed tips 250, the thermal conductivity can be reduced to aminimum and produce a figure of merit, ZT, for these materials ofgreater than 4, thus making these materials very attractive for use inthermoelements 210 and 212. Therefore, the use of pointed tips 250further increases the efficiency of the thermoelectric cooler 200 suchthat it is comparable to vapor compression refrigerators.

[0043] Another advantage of the cold point structure is that theelectrons are confined to dimensions smaller than the wavelength(corresponding to their kinetic energy). This type of confinementincreases the local density of states available for transport andeffectively increases the Seebeck coefficient. Thus, by increasing α anddecreasing λ, the figure of merit ZT is increased.

[0044] Normal cooling capacity of conventional thermoelectric coolers,such as illustrated in FIG. 1, are capable of producing a temperaturedifferential, ΔT, between the heat source and the heat sink of around 60Kelvin. However, thermoelectric cooler 200 is capable of producing atemperature differential on the order of 150 Kelvin. Thus, with twothermoelectric coolers coupled to each other, cooling to temperatures inthe range of liquid Nitrogen (less than 100 Kelvin) is possible.However, different materials may need to be used for thermoelements 210and 212. For example, bismuth telluride has a very low a at lowtemperature (i.e. less than −100 degrees Celsius). However, bismuthantimony alloys perform well at low temperature.

[0045] Another advantage of the cobalt antimony skutteridite materialsover the bismuth chalcogenide materials, not related to temperature, isthe fact the cobalt antimony skutteridite materials are structurallymore stable whereas the bismuth chalcogenide materials are structurallyweak.

[0046] Those of ordinary skill in the art will appreciate that theconstruction of the thermoelectric cooler in FIG. 2 may vary dependingon the implementation. For example, more or fewer rows of tips 250 maybe included than depicted in FIG. 1. The depicted example is not meantto imply architectural limitations with respect to the presentinvention.

[0047] With reference now to FIG. 3, a planer view of thermoelectriccooler 200 in FIG. 2 is depicted in accordance with the presentinvention. Thermoelectric cooler 300 includes an n-type thermoelectricmaterial section 302 and a p-type thermoelectric material section 304.Both n-type section 302 and p-type section 304 include a thin layer ofconductive material 306 that covers a silicon body.

[0048] Section 302 includes an array of conical tips 310 each coveredwith a thin layer of n-type material 308 of the same type as the nearestlayer of the superlattice for thermoelement 210. Section 304 includes anarray of conical tips 312 each covered with a thin layer of p-typematerial 314 of the same type as the nearest layer of the superlatticefor thermoelement 212.

[0049] With reference now to FIGS. 4A and 4B, a cross sectional views oftips that may be implemented as one of tips 250 in FIG. 2 is depicted inaccordance with the present invention. Tip 400 includes a silicon conethat has been formed with a cone angle of approximately 35 degrees. Athin layer 404 of conducting material, such as platinum (Pt), overcoatsthe silicon 402. A thin layer of thermoelectric material 406 covers thevery end of the tip 400. The cone angle after all layers have beendeposited is approximately 45 degrees. The effective tip radius of tip400 is approximately 50 nanometers.

[0050] Tip 408 is an alternative embodiment of a tip, such as one oftips 250. Tip 408 includes a silicon cone 414 with a conductive layer412 and thermoelectric material layer 410 over the point. However, tip408 has a much sharper cone angle than tip 400. The effective tip radiusof tip 408 is approximately 10 nanometers. It is not known at this timewhether a broader or narrower cone angle for the tip is preferable. Inthe present embodiment, conical angles of 45 degrees for the tip, asdepicted in FIG. 4A, have been chosen, since such angle is in the middleof possible ranges of cone angle and because such formation is easilyformed with silicon with a platinum overcoat. This is because a KOH etchalong the 100 plane of silicon naturally forms a cone angle of 54degrees. Thus, after the conductive and thermoelectric overcoats havebeen added, the cone angle is approximately 45 degrees.

[0051] With reference now to FIG. 5, a cross sectional view illustratingthe temperature field of a tip near to a superlattice is depicted inaccordance with the present invention. Tip 504 may be implemented as oneof tips 250 in FIG. 2. Tip 504 has a effective tip radius, a, of 30-50nanometers. Thus, the temperature field is localized to a very smalldistance, r, approximately equal to 2a or around 60-100 nanometers.Therefore, a superlattice 502 need to be only a few layers thick with athickness, d, of around 100 nanometers. Therefore, using pointed tips, athermoelectric cooler with only 5-10 layers for the superlattice issufficient.

[0052] Thus, fabricating a thermoelectric cooler, such as, for example,thermoelectric cooler 200, is not extremely time consuming, since only afew layers of the superlattice must be formed rather than numerouslayers which can be very time consuming. Thus, thermoelectric cooler 200can be fabricated very thin (on the order of 100 nanometers thick) ascontrasted to prior art thermoelectric coolers which were on the orderof 3 millimeters or greater in thickness.

[0053] Other advantages of a thermoelectric cooler with pointed tipinterfaces in accordance with the present invention include minimizationof the thermal conductivity of the thermoelements, such asthermoelements 210 and 212 in FIG. 2, at the tip interfaces. Also, thetemperature/potential drops are localized to an area near the tips,effectively achieving scaling to sub-100-nanometer lengths. Furthermore,using pointed tips minimizes the number layers for superlattice growthby effectively reducing the thermoelement lengths. The present inventionalso permits electrodeposition of thin film structures and avoidsflip-chip bonds. The smaller dimensions allow for monolithic integrationof n-type and p-type thermoelements.

[0054] The thermoelectric cooler of the present invention may beutilized to cool items, such as, for example, specific spots within amain frame computer, lasers, optic electronics, photodetectors, and PCRin genetics.

[0055] With reference now to FIG. 6, a cross sectional view of athermoelectric cooler with enhanced structured interfaces with all metaltips is depicted in accordance with the present invention. Although thepresent invention has been described above as having tips 250constructed from silicon cones constructed from the n+ semiconductingregions 224 and 222, tips 250 in FIG. 2 may be replaced by tips 650 asdepicted in FIG. 6. Tips 650 have all metal cones 618 and 620. In thedepicted embodiment, cones 618 and 620 are constructed from copper andhave a nickel overcoat layer 660 and 662. Thermoelectric cooler 600 isidentical to thermoelectric cooler 200 in all other respects, includinghaving a thermoelectric overcoat 216 and 214 over the tips 650.Thermoelectric cooler 600 also provides the same benefits asthermoelectric cooler 200. However, by using all metal cones rather thansilicon cones covered with conducting material, the parasiticresistances within the cones become very low, thus further increasingthe efficiency of thermoelectric cooler 600 over the already increasedefficiency of thermoelectric cooler 200. The areas surrounding tips 650and between tips 650 and thermoelectric materials 210 and 212 should bevacuum or hermetically sealed with a gas, such as, for example, drynitrogen.

[0056] Also, as in FIG. 2, heat source 226 is comprised of p− type dopedsilicon. In contrast to FIG. 2, however, heat source 226 is thermallycoupled to n+ type doped silicon regions 624 and 622 that do not formpart of the tipped structure 650 rather than to regions that do formpart of the tipped structure as do regions 224 and 222 do in FIG. 2. N+type doped silicon regions 624 and 622 do still perform the electricalisolation function performed by regions 224 and 222 in FIG. 2.

[0057] Several methods may be utilized to form the all metal cones asdepicted in FIG. 6. For example, with reference now to FIG. 7, across-sectional view of a sacrificial silicon template that may be usedfor forming all metal tips is depicted in accordance with the presentinvention. After the sacrificial silicon template 702 has beenconstructed having conical pits, a layer of metal may be deposited overthe template 702 to produce all metal cones 704. All metal cones 704 maythen be used in thermoelectric cooler 600.

[0058] With reference now to FIG. 8, a flowchart illustrating anexemplary method of producing all metal cones using a siliconsacrificial template is depicted in accordance with the presentinvention. To begin, conical pits are fabricated by anisotropic etchingof silicon to create a mold (step 802). This may be done by acombination of KOH etching, oxidation, and/or focused ion-beam etching.Such techniques of fabricating a silicon cone are well known in the art.

[0059] The silicon sacrificial template is then coated with a thinsputtered layer of seed metal, such as, for example, titanium orplatinum (step 804). Titanium is preferable since platinum formsslightly more rounded tips than titanium, which is very conforming tothe conical pits. Next, copper is electrochemically deposited to fillthe valleys (conical pits) in the sacrificial silicon template. (step806). The top surface of the copper is then planarized (step 808).Methods of planarizing a layer of metal are well known in the art. Thesilicon oxide (SiO₂) substrate is then removed by selective etchingmethods well known in the art (step 810). The all metal cones producedin this manner may then be covered with a coat of another metal, suchas, for example, nickel or titanium and then with an ultra-thin layer ofthermoelectric material. The nickel or titanium overcoat aids inelectrodeposition of the thermoelectric material overcoat.

[0060] One advantage to this method of producing all metal cones is thatthe mold that is produced is reusable. The mold may be reused up toaround 10 times before the mold degrades and becomes unusable. Forming atemplate in this manner is very well controlled and produces veryuniform all metal conical tips since silicon etching is very predictableand can calculate slopes of the pits and sharpness of the cones producedto a very few nanometers.

[0061] Other methods of forming all metal cones may be used as well. Forexample, with reference now to FIG. 9, a cross sectional view of allmetal cones 902 formed using patterned photoresist is depicted inaccordance with the present invention. In this method, a layer of metalis formed over the bottom portions of a partially fabricatedthermoelectric cooler. A patterned photoresist 904-908 is then used tofashion all metal cones 902 with a direct electrochemical etchingmethod.

[0062] With reference now to FIG. 10, a flowchart illustrating anexemplary method of forming all metal cones using photoresist isdepicted in accordance with the present invention. To begin, smallsections of photoresist are patterned over a metal layer, such ascopper, of a partially fabricated thermoelectric cooler, such asthermoelectric cooler 600, in FIG. 6 (step 1002). The photoresist may bepatterned in an array of sections having photoresist wherein each areaof photoresist within the array corresponds to areas in which tips tothe all metal cones are desired to be formed. The metal is then directlyetched electrochemically (step 1004) to produce cones 902 as depicted inFIG. 9. The photoresist is then removed and the tips of the all metalcones may then be coated with another metal, such as, for example,nickel (step 1006). The second metal coating over the all metal conesmay then be coated with an ultra-thin layer of thermoelectric material(step 1008). Thus, all metal cones with a thermoelectric layer on thetips may be formed which may used in a thermoelectric device, such as,for example, thermoelectric cooler 600. The all metal conical pointsproduced in this manner are not as uniform as those produced using themethod illustrated in FIG. 8. However, this method currently is cheaperand therefore, if cost is an important factor, may be a more desirablemethod.

[0063] The depicted methods of fabricating all metal cones are merelyexamples. Other methods may be used as well to fabricate all metal conesfor use with thermoelectric coolers. Furthermore, other types of metalsmay be used for the all metal cone other than copper.

[0064] With reference now to FIG. 11, a cross-sectional view of athermoelectric cooler with enhanced structural interfaces in which thethermoelectric material rather than the metal conducting layer is formedinto tips at the interface is depicted in accordance with the presentinvention. Thermoelectric cooler 1100 includes a cold plate 1116 and ahot plate 1102, wherein the cold plate is in thermal contact with thesubstance that is to be cooled. Thermal conductors 1114 and 1118 providea thermal couple between electrical conducting plates 1112 and 1120respectively. Thermal conductors 1114 and 1118 are constructed ofheavily n doped (n+) semiconducting material that provides electricalisolation between cold plate 1116 and conductors 1112 and 1120 byforming reverse biased diodes with the p− material of the cold plate1116. Thus, heat is transferred from the cold plate 1116 throughconductors 1112 and 1120 and eventually to hot plate 1102 from which itcan be dissipated without allowing an electrical coupling between thethermoelectric cooler 1100 and the substance that is to be cooled.Similarly, thermal conductor 1104 provides a thermal connection betweenelectrical conducting plate 1108 and hot plate 1102, while maintainingelectrical isolation between the hot plate and electrical conductingplate 1108 by forming a reverse biased diode with the hot plate 1102 p−doped semiconducting material as discussed above. Thermal conductors1104 and 1106 are also an n+ type doped semiconducting material.Electrical conducting plates 1108, 1112, and 1120 are constructed fromplatinum (Pt) in this embodiment. However, other materials that are bothelectrically conducting and thermally conducting may be utilized aswell. Also, it should be mentioned that the areas surrounding tips1130-1140 and between tips 1130-1140 and thermoelectric materials 1122and 1110 should be evacuated to produce a vacuum or should behermetically sealed with a gas, such as, for example, dry nitrogen.

[0065] In this embodiment, rather than providing contact between thethermoelements and the heat source (cold end) metal electrode(conductor) through an array of points in the metal electrode as inFIGS. 2 and 6, the array of points of contact between the thermoelementand the metal electrode is provided by an array of points 1130-1140 inthe thermoelements 1124 and 1126. In the embodiments described abovewith reference to FIGS. 2 and 6, the metal electrode at the cold end wasformed over silicon tips or alternatively metal patterns were directlyetched to form all-metal tips. However, these methods requiredthermoelectric materials to be deposited over the cold and the hotelectrodes by electrochemical methods. The electrodeposited materialstend to be polycrystalline and do not have ultra-planar surfaces. Also,the surface thermoelectric properties may or may not be superior tosingle crystalline thermoelectric materials. Annealing improves thethermoelectric properties of the polycrystalline materials, but surfacesmoothness below 100 nm roughness levels remains a problem. The tips1130-1140 of the present embodiment may be formed from single crystal orpolycrystal thermoelectric materials by electrochemical etching.

[0066] In one embodiment, thermoelement 1124 is comprised of a superlattice of single crystalline Bi₂Te₃/Sb₂Te₃ and Bi_(0.5)Sb_(1.5)Te₃ andthermoelement 1126 is formed of a super lattice of single crystallineBi₂Te₃/Bi₂Se₃ and Bi₂Te_(2.0)Se_(0.1). Electrically conducting plate1120 is coated with a thin layer 1122 of the same thermoelectricmaterial as is the material of the tips 1130-1134 that are nearest thinlayer 1120. Electrically conducting plate 1112 is coated with a thinlayer 1110 of the same thermoelectric material as is the material of thetips 1136-1140 that are nearest thin layer 1112.

[0067] With reference now to FIG. 12, a flowchart illustrating anexemplary method of fabricating a thermoelectric cooler, such as, forexample, thermoelectric cooler 1100 in FIG. 11, is depicted inaccordance with the present invention. Optimized single crystal materialare first bonded to metal electrodes by conventional means or metalelectrodes are deposited onto single crystal materials to form theelectrode connection pattern (step 1202). The other side of thethermoelectric material 1314 is then patterned (step 1204) byphotoresist 1302-1306 as depicted in FIG. 13 and metal electrodes areused in an electrochemical bath as an anode to electrochemically etchthe surface (step 1206). The tips 1308-1312 as depicted in FIG. 13 areformed by controlling and stopping the etching process at appropriatetimes.

[0068] A second single crystal substrate is thinned bychemical-mechanical polishing and then electrochemically etching theentire substrate to nanometer films (step 1210). The second substratewith the ultra-thin substrate forms the cold end and the two substrates(the one with the ultra-thin thermoelectric material and the other withthe thermoelectric tips) are clamped together with pressure (step 1212).This structure retains high crystallinity in all regions other than theinterface at the tips. Also, the same method can be used to fabricatepolycrystalline structures rather than single crystalline structures.

[0069] With reference now to FIG. 14, a diagram showing a cold point tipabove a surface for use in a thermoelectric cooler illustrating thepositioning of the tip relative to the surface is depicted in accordancewith the present invention. Although the tips, whether created in asall-metal or metal coated tips or as thermoelectric tips have beendescribed thus far as being in contact with the surface opposite thetips. However, although the tips may be in contact with the opposingsurface, it is preferable that the tips be near the opposing surfacewithout touching the surface as depicted in FIG. 14. The tip 1402 inFIG. 14 is situated near the opposing surface 1404 but is not inphysical contact with the opposing surface. Preferably, the tip 1402should be a distance d on the order of 5 nanometers or less from theopposing surface 1404. In practice, with a thermoelectric coolercontaining thousands of tips, some of the tips may be in contact withthe opposing surface while others are not in contact due to thedeviations from a perfect plane of the opposing surface.

[0070] By removing the tips from contact with the opposing surface, theamount of thermal conductivity between the cold plate and the hot plateof a thermoelectric cooler may be reduced. Electrical conductivity ismaintained, however, due to tunneling of electrons between the tips andthe opposing surface.

[0071] The tips of the present invention have also been described anddepicted primarily as perfectly pointed tips. However, as illustrated inFIG. 14, the tips in practice will typically have a slightly morerounded tip as is the case with tip 1402. However, the closer toperfectly pointed the tip is, the fewer number of superlattices neededto achieve the temperature gradient between the cool temperature of thetip and the hot temperature of the hot plate.

[0072] Preferably, the radius of curvature r₀ of the curved end of thetip 1402 is on the order of a few tens of nanometers. The temperaturedifference between adjacent areas of the thermoelectric material belowsurface 1404 approaches zero over a distance of two (2) to three (3)times the radius of curvature r₀ of the end of tip 1402. Therefore, onlya few layers of the super lattice 1406-1414 are necessary. Thus, asuperlattice material opposite the tips is feasible when the electricalcontact between the hot and cold plates is made using the tips of thepresent invention. This is in contrast to the prior art in which to usea superlattice structure without tips, a superlattice of 10000 or morelayers was needed to have a sufficient thickness in which to allow thetemperature gradient to approach zero. Such a number of layers wasimpractical, but using only 5 or 6 layers as in the present invention ismuch more practical.

[0073] Although the present invention has been described primarily withreference to a thermoelectric cooling device (or Peltier device) withtipped interfaces used for cooling, it will be recognized by thoseskilled in the art that the present invention may be utilized forgeneration of electricity as well. It is well recognized by thoseskilled in the art that thermoelectric devices can be used either in thePeltier mode (as described above) for refrigeration or in the Seebeckmode for electrical power generation. Referring now to FIG. 15, aschematic diagram of a thermoelectric power generator is depicted. Forease of understanding and explanation of thermoelectric powergeneration, a thermoelectric power generator according to the prior artis depicted rather than a thermoelectric power generator utilizing coolpoint tips of the present invention. However, it should be noted that inone embodiment of a thermoelectric power generator according to thepresent invention, the thermoelements 1506 and 1504 are replaced coolpoint tips, as for example, any of the cool point tip embodiments asdescribed in greater detail above.

[0074] In a thermoelectric power generator 1500, rather than runningcurrent through the thermoelectric device from a power source 102 asindicated in FIG. 1, a temperature differential, T_(H)-T_(L), is createdacross the thermoelectric device 1500. Such temperature differential,T_(H)-T_(L), induces a current flow, I, as indicated in FIG. 15 througha resistive load element 1502. This is the opposite mode of operationfrom the mode of operation described in FIG. 1

[0075] Therefore, other than replacing a power source 102 with aresistor 1502 and maintaining heat elements 1512 and 1516 and constanttemperatures T_(H) and T_(L) respectively with heat sources Q_(H) andQ_(L) respectively, thermoelectric device 1500 is identical incomponents to thermoelectric device 102 in FIG. 1. Thus, thermoelectriccooling device 1500 utilizes p-type semiconductor 1504 and n-typesemiconductor 1506 sandwiched between poor electrical conductors 1508that have good heat conducting properties. Each of elements 1504, 1506,and 1508 correspond to elements 104, 106, and 108 respectively inFIG. 1. Thermoelectric device 1500 also includes electrical conductors1510 and 1514 corresponding to electrical conductors 110 and 114 inFIG. 1. More information about thermoelectric electric power generationmay be found in CRC Handbook of Thermoelectrics, edited by D. M. Rowe,Ph.D., D.Sc., CRC Press, New York, (1995) pp. 479-488 and in AdvancedEngineering Thermodynamics, 2nd Edition, by Adiran Bejan, John Wiley &Sons, Inc., New York (1997), pp. 675-682, both of which are herebyincorporated herein for all purposes.

[0076] The present invention has been described primarily with referenceto conically shaped tips, however, other shapes of tips may be utilizedas well, such as, for example, pyramidically shaped tips. In fact, theshape of the tip does not need to be symmetric or uniform as long as itprovides a discrete set of substantially pointed tips through whichelectrical conduction between the two ends of a thermoelectric coolermay be provided. The present invention has applications to use in anysmall refrigeration application, such as, for example, cooling mainframe computers, thermal management of hot chips and RF communicationcircuits, cooling magnetic heads for disk drives, automobilerefrigeration, and cooling optical and laser devices.

[0077] The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A thermoelectric device, comprising: a firstthermoelement constructed from a first type of thermoelectric material;a second thermoelement, constructed from a second type of thermoelectricmaterial, electrically coupled to the first thermoelement; an array offirst tips proximate to the first thermoelement at a first set ofdiscrete points such that electrical conduction between the array offirst tips and the first thermoelement is facilitated but thermalconduction between the array of first tips and the first thermoelementis retarded; and an array of second tips proximate to the secondthermoelement at a second set of discrete points such that electricalconduction between the array of second tips and the second thermoelementis facilitated while thermal conduction between the array of second tipsand the second thermoelement is retarded; wherein the first and secondtips are constructed from metal.
 2. The thermoelectric device as recitedin claim 1, wherein the metal is copper.
 3. The thermoelectric device asrecited in claim 1, wherein the metal is a first metal and furthercomprising: a second layer of metal overcoating each of the first andsecond conical tips.
 4. The thermoelectric device as recited in claim 3,wherein the second layer of metal comprises nickel.
 5. Thethermoelectric device as recited in claim 1, further comprising: layersof thermoelectric material overcoating each of the first and secondconical tips, wherein the thermoelectric material layer impurity typematch the respective impurity types of the proximate the first andsecond thermoelements.
 6. The thermoelectric device as recited in claim1, wherein the first and second thermoelements each comprise a first anda second superlattice of thermoelectric material respectively.
 7. Thethermoelectric device as recited in claim 1, wherein the first andsecond tips are substantially conical.
 8. The thermoelectric device asrecited in claim 1, wherein the first and second tips are substantiallypyramidically shaped.
 9. A method of forming all metal tips for use in athermoelectric device, the method comprising: fabricating a planarsacrificial template with a pitted surface having multiple valleys ofconsistent depth; covering the sacrificial template with a layer ofmetal extending into the valleys of the sacrificial template; andremoving the sacrificial template to create a layer of metal withmultiple tips.
 10. The method as recited in claim 9, wherein the tipsare conical in shape.
 11. The method as recited in claim 9, wherein thetips are pyramid in shape.
 12. A method of forming metal pointed tipsfor use in a thermoelectric device, the method comprising: forming amask of patterned photoresist onto a layer of metal; etching the layerof metal in the presence of the photoresist mask to producesubstantially pointed tipped structures of metal; and removing thephotoresist.
 13. The method as recited in claim 12, wherein thepatterned photoresist forms an array of photoresist areas thatcorrespond to areas for which tips of the substantially pointed tippedstructures of metal are desired.
 14. The method as recited in claim 12,wherein the metal is copper.
 15. The method as recited in claim 12,further comprising: coating the substantially pointed tipped structuresof metal with a layer of a second metal.
 16. The method as recited inclaim 12, further comprising: coating the substantially pointed tippedstructures of metal with a layer of thermoelectric material.
 17. Themethod as recited in claim 15, further comprising: coating the layer ofsecond metal with a layer of thermoelectric material.
 18. The method asrecited in claim 12, wherein the substantially pointed tipped structuresare conical shaped.
 19. The method as recited in claim 12, wherein thesubstantially pointed tipped structures are pyramid shaped.
 20. A systemof forming metal pointed tips for use in a thermoelectric device, thesystem comprising: means for forming a mask of patterned photoresistonto a layer of metal; means for etching the layer of metal in thepresence of the photoresist mask to produce substantially pointed tippedstructures of metal; and means for removing the photoresist.
 21. Thesystem as recited in claim 20, wherein the patterned photoresist formsan array of photoresist areas that correspond to areas for which tips ofthe substantially pointed tipped structures of metal are desired. 22.The system as recited in claim 20, wherein the metal is copper.
 23. thesystem as recited in claim 20, further comprising: means for coating thesubstantially pointed tipped structures of metal with a layer of asecond metal.
 24. The system as recited in claim 20, further comprising:means for coating the substantially pointed tipped structures of metalwith a layer of thermoelectric material.
 25. The system as recited inclaim 23, further comprising: means for coating the layer of secondmetal with a layer of thermoelectric material.
 26. The system as recitedin claim 20, wherein the substantially pointed tipped structures areconical shaped.
 27. The system as recited in claim 20, wherein thesubstantially pointed tipped structures are pyramid shaped.